High-speed, high-resolution analog-to-digital converters (ADCs) have been the subject of much research for years. Such systems would allow for digitization directly from the output of a broadband antenna, the implementation of software defined radio into higher bands of operation, and many other measurement, control and communications applications.
Photonic ADCs have been considered. An overview of the state-of-the-art in photonic ADCs as of 2007 can be found in George C. Valley, Photonic analog-to-digital converters, OPTICS EXPRESS, Vol. 15, No. 5 Mar. 2007, pp. 1955-1982.
Some prior approaches to the design of a photonic ADC use electro-optic modulators to achieve speed, but sometimes in impractical ways. For example, a fundamental design parameter of an electro-optic cell is its interaction length L. Some approaches for an N-bit ADC use modulators with interaction lengths that vary as L, 2L, 4L, . . . , 2NL. This practically limits the ADC resolution to 4 to 6 bits, since achieving this interaction length disparity would require a prohibitively high system complexity, size and cost for a large number of bits N. Further, the longer the cell the slower its speed. The loss also becomes impractically large for applications with long interaction lengths.
Other prior approaches avoid the interaction length disparity by using modulators of a fixed length. But an N-bit ADC requires a large quantity (2N) of such modulators with the RF signal driving each of them. Therefore, these approaches are only practical for applications with a small number of bits, with for example N of 6 or less.